Messages in this thread |  | | | Date | Sun, 4 Sep 2011 16:36:43 -0700 | | From | "Paul E. McKenney" <> | | Subject | Re: [PATCH 1/4] nohz: Split extended quiescent state handling from nohz switch |
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On Sat, Aug 20, 2011 at 07:30:49PM +0200, Frederic Weisbecker wrote: > It is assumed that rcu won't be used once we switch to tickless > mode and until we restart the tick. However this is not always > true, as in x86-64 where we dereference the idle notifiers after > the tick is stopped. > > To prepare for fixing this, split the tickless mode switching and > RCU extended quiescent state logics. > Make tick_nohz_stop/restart_sched_tick() RCU agnostic but provide > a new pair of APIs tick_nohz_enter/exit_idle() that keep the > old behaviour by handling both the nohz mode and RCU extended > quiescent states, then convert every archs to use these. > > Archs that want to switch to RCU extended QS to some custom points > can do it later by changing the parameter in tick_nohz_enter,exit_idle() > to false and call rcu_enter,exit() separately.
This approach looks quite good to me! A few comments below.
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> > Cc: David Miller <davem@davemloft.net> > Cc: Chris Metcalf <cmetcalf@tilera.com> > Cc: Guan Xuetao <gxt@mprc.pku.edu.cn> > Cc: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com> > Cc: Mike Frysinger <vapier@gentoo.org> > Cc: Ralf Baechle <ralf@linux-mips.org> > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > Cc: Ingo Molnar <mingo@elte.hu> > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: H. Peter Anvin <hpa@zytor.com> > Cc: Russell King <linux@arm.linux.org.uk> > Cc: Paul Mackerras <paulus@samba.org> > Cc: Heiko Carstens <heiko.carstens@de.ibm.com> > Cc: Paul Mundt <lethal@linux-sh.org> > ---
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> diff --git a/arch/powerpc/kernel/idle.c b/arch/powerpc/kernel/idle.c > index 39a2baa..b30ddf1 100644 > --- a/arch/powerpc/kernel/idle.c > +++ b/arch/powerpc/kernel/idle.c > @@ -56,7 +56,7 @@ void cpu_idle(void) > > set_thread_flag(TIF_POLLING_NRFLAG); > while (1) { > - tick_nohz_stop_sched_tick(1); > + tick_nohz_enter_idle(true);
This needs to pass false, as some variants of powerpc use event tracing (which in turn uses RCU) just before returning to the hypervisor.
> while (!need_resched() && !cpu_should_die()) { > ppc64_runlatch_off(); > > @@ -93,7 +93,7 @@ void cpu_idle(void) > > HMT_medium(); > ppc64_runlatch_on(); > - tick_nohz_restart_sched_tick(); > + tick_nohz_exit_idle(true);
As does this, as some variants of powerpc use event tracing just after the hypervisor returns control to the OS.
> preempt_enable_no_resched(); > if (cpu_should_die()) > cpu_die(); > diff --git a/arch/powerpc/platforms/iseries/setup.c b/arch/powerpc/platforms/iseries/setup.c > index c25a081..aa2be7d 100644 > --- a/arch/powerpc/platforms/iseries/setup.c > +++ b/arch/powerpc/platforms/iseries/setup.c > @@ -562,7 +562,7 @@ static void yield_shared_processor(void) > static void iseries_shared_idle(void) > { > while (1) { > - tick_nohz_stop_sched_tick(1); > + tick_nohz_enter_idle(true);
But I don't know enough about iseries to know whether or not this works.
> while (!need_resched() && !hvlpevent_is_pending()) { > local_irq_disable(); > ppc64_runlatch_off(); > @@ -576,7 +576,7 @@ static void iseries_shared_idle(void) > } > > ppc64_runlatch_on(); > - tick_nohz_restart_sched_tick(); > + tick_nohz_exit_idle(true); > > if (hvlpevent_is_pending()) > process_iSeries_events(); > @@ -592,7 +592,7 @@ static void iseries_dedicated_idle(void) > set_thread_flag(TIF_POLLING_NRFLAG); > > while (1) { > - tick_nohz_stop_sched_tick(1); > + tick_nohz_enter_idle(true); > if (!need_resched()) { > while (!need_resched()) { > ppc64_runlatch_off(); > @@ -609,7 +609,7 @@ static void iseries_dedicated_idle(void) > } > > ppc64_runlatch_on(); > - tick_nohz_restart_sched_tick(); > + tick_nohz_exit_idle(true); > preempt_enable_no_resched(); > schedule(); > preempt_disable();
[ . . . ]
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > index eb98e55..609bb20 100644 > --- a/kernel/time/tick-sched.c > +++ b/kernel/time/tick-sched.c > @@ -253,12 +253,13 @@ EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us); > * Called either from the idle loop or from irq_exit() when an idle period was > * just interrupted by an interrupt which did not cause a reschedule. > */ > -void tick_nohz_stop_sched_tick(int inidle) > +bool tick_nohz_stop_sched_tick(int inidle) > { > unsigned long seq, last_jiffies, next_jiffies, delta_jiffies, flags; > struct tick_sched *ts; > ktime_t last_update, expires, now; > struct clock_event_device *dev = __get_cpu_var(tick_cpu_device).evtdev; > + int stopped = false; > u64 time_delta; > int cpu; > > @@ -405,7 +406,7 @@ void tick_nohz_stop_sched_tick(int inidle) > ts->idle_tick = hrtimer_get_expires(&ts->sched_timer); > ts->tick_stopped = 1; > ts->idle_jiffies = last_jiffies; > - rcu_enter_nohz(); > + stopped = true; > } > > ts->idle_sleeps++; > @@ -445,6 +446,24 @@ out: > ts->sleep_length = ktime_sub(dev->next_event, now); > end: > local_irq_restore(flags); > + > + return stopped; > +} > + > + > +/** > + * tick_nohz_enter_idle - stop the tick from the idle task > + * @rcu_ext_qs: enter rcu dynticks idle mode > + * > + * When an arch doesn't make any use of rcu read side critical section > + * between tick_nohz_enter_idle() and tick_nohz_exit_idle() it can set > + * rcu_ext_qs to 1. Otherwise it needs to call rcu_enter_nohz() itself > + * later before the CPU goes to sleep.
How about something like this?
+ * When an arch doesn't make any use of rcu read side critical section + * between tick_nohz_enter_idle() and the time the CPU is put to sleep, + * it can set rcu_ext_qs to true. Otherwise it needs set rcu_ext_qs to + * false, and then to call rcu_enter_nohz() itself later, but before the + * CPU goes to sleep and after its last use of RCU.
> + */ > +void tick_nohz_enter_idle(bool rcu_ext_qs) > +{ > + if (tick_nohz_stop_sched_tick(1) && rcu_ext_qs) > + rcu_enter_nohz(); > } > > /** > @@ -486,11 +505,12 @@ static void tick_nohz_restart(struct tick_sched *ts, ktime_t now) > } > > /** > - * tick_nohz_restart_sched_tick - restart the idle tick from the idle task > + * tick_nohz_exit_idle - restart the idle tick from the idle task > + * @rcu_ext_qs: exit rcu dynticks idle mode > * > * Restart the idle tick when the CPU is woken up from idle
How about something like the following?
+ * When an arch doesn't make any use of rcu read side critical section + * between the time the CPU wakes up and tick_nohz_exit_idle(), it can set + * rcu_ext_qs to true. Otherwise it needs set rcu_ext_qs to false, and it + * must also have called rcu_enter_nohz() itself earlier, after the CPU + * was awakened, but before its first use of RCU.
> */ > -void tick_nohz_restart_sched_tick(void) > +void tick_nohz_exit_idle(bool rcu_ext_qs) > { > int cpu = smp_processor_id(); > struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu); > @@ -514,7 +534,8 @@ void tick_nohz_restart_sched_tick(void) > > ts->inidle = 0; > > - rcu_exit_nohz(); > + if (rcu_ext_qs) > + rcu_exit_nohz(); > > /* Update jiffies first */ > select_nohz_load_balancer(0); > -- > 1.7.5.4 >
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